1. Field of the Invention
The present invention relates to a superconductive electronic device which provides ultrafast switching performance in a high-speed digital or analog data processing circuits, and more particularly to a configuration of a superconductor signal amplifier required for a superconductive circuit using flux quanta as binary information carriers.
2. Description of the Related Art
A superconductive single flux quantum (SFQ) circuit, in which an SFQ is used as an information carrier, i.e., the presence/absence of an SFQ is used to represent binary xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, is an extremely high-speed circuit capable of transmitting signals having a width of the order of picoseconds at a frequency exceeding 100 GHz. Considerable research has been performed on SFQ circuit applications to realize an analog-digital converter circuit, digital logic circuit, or the like that has ultrafast operational performance. A signal amplitude in the SFQ circuit, however, is so extremely small that an output voltage as low as 1 mV or less is attainable. Although such a low signal level is advantageous in that power consumption and heat generation are low in circuit operation, connections to common semiconductor circuits such as CMOS and ECL are required in practical use of the SFQ circuit, and it is therefore necessary to amplify an output voltage of the SFQ circuit up to an operating voltage level of each circuit which receives an output signal from the SFQ circuit. Moreover, where electrical wiring is provided from the SFQ circuit placed in a cryogenic container to a circuit in a room-temperature environment, an output signal is liable to attenuation due to necessity of relatively long wiring arrangement. From this point of view, it is also necessary to provide a signal amplifier circuit at an output part of the SFQ circuit.
Conventional superconductor signal amplifier circuits are classified into the following two types; a signal amplifier in which series-connected Josephson junctions are simultaneously switched to a voltage state, and a signal amplifier which uses a superconducting quantum interference device (SQUID). The SQUID is a superconductive device comprising two Josephson junctions disposed in a closed loop formed of a superconductive wiring material, a bias current input terminal disposed between the two Josephson junctions, and a ground terminal disposed on the opposite side thereof, wherein a large change in voltage is produced across the two terminals by applying extremely small variation in magnetic field.
An example of a circuit for amplifying a very-high-speed signal through the use of SQUIDs has been proposed by O. A. Mukhanov et al. in IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, p. 2826, FIG. 1, 1997. In this example, a Josephson transmission line is so arranged that a flux quantum signal is split into a plurality of flux quantum signals while maintaining a pulse signal characteristic thereof, and each of the plural flux quantum signals is input to a control line for applying a magnetic field to each of a plurality of series-connected SQUIDs disposed in an array. On the Josephson transmission line, two or more Josephson junctions are parallel-connected using superconductive wiring, and the one end thereof is connected to ground and the other end thereof is connected to a bias source for flux quantum signal propagation through the superconductive wiring. On input of a flux quantum signal, the Josephson junctions are switched in succession so that the flux quantum signal propagates while reshaping a pulse waveform thereof.
In the above arrangement in which the splitting of a flux quantum signal is made, the control line for applying a magnetic field to SQUID has a length shorter than that in a single-control-line routing arrangement, thereby reducing a signal delay which is proportional to the inductance of the control line. Thus, an 8 GHz signal is amplified to 2 mV.
Another example of a circuit using SQUIDS, though intended for a different purpose, has been reported by S. Polonsky et al. in IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, p. 2818, 1997. In signal transfer between superconductive chips, it is required to avoid impedance mismatching due to propagation through wiring between the chips on a substrate. For this purpose, the above-noted circuit is designed to convert a single flux quantum signal into four multi-flux quantum signals. In the same report (FIG. 3), there is disclosed a circuit scheme in which splitters are cascaded to perform simultaneous switching of SQUIDs.
It is an object of the present invention to provide a superconductor signal amplifier for amplifying a flux quantum signal having a frequency of tens or hundreds of GHz from a single flux quantum circuit. More specifically, the present invention provides a superconductor signal amplifier which is capable of performing sufficient amplification while maintaining high speed and enhancing responsivity to an input signal.
In carrying out the present invention and according to one aspect thereof, there is provided a superconductor signal amplifier circuit comprising a plurality of series-connected SQUIDs (superconducting quantum interference devices), in which flux quantum signals, each maintaining a pulse characteristic, are input simultaneously to a control line for applying a magnetic field to each of the SQUIDs so that magnetic coupling is made on the SQUIDs, thereby simultaneously switching the SQUIDs to a voltage state. To be more specific, there is provided a superconducting quantum interference device comprising: a first Josephson junction pair made of two Josephson junctions; first and second inductive elements connected to the first Josephson junction pair in a loop configuration to form a first superconductive loop; a second Josephson junction pair made of two Josephson junctions connected in a loop configuration for sharing a part of the second inductive element to form a second superconductive loop; a third inductive element connected to the second Josephson junction pair in a loop configuration on the second superconductive loop; and an input circuit magnetically coupled with the first and third inductive elements.
Further, according to another aspect of the present invention, there is provided a superconductive circuit comprising: a first Josephson transmission line including; an inductive element having an input terminal for signal input and an output terminal for signal output, Josephson junctions respectively connected to the input and output terminals of the inductive element, and direct current sources respectively connected to the input and output terminals of the inductive element; a second Josephson transmission line which is identical in structure to the first Josephson transmission line; a fourth inductive element connected between an input terminal of the second Josephson transmission line and an output terminal of the first Josephson transmission line; a first inductive element magnetically coupled with the fourth inductive element; a Josephson junction pair made of two Josephson junctions; a first superconductive loop connected to the Josephson junction pair and the forth inductive element in a loop configuration; and a resistor element connected to an output terminal of the second Josephson transmission line.
Still further, according to another aspect of the present invention, there is provided a superconductive circuit comprising: a plurality of superconductive circuits each including; a substrate, a magnetic shielding film formed on the substrate, a first insulating film for electrical insulation on the magnetic shielding film, a first superconductive thin film formed on the first insulating film, a second insulating film formed on the first superconductive thin film, a second superconductive thin film formed on the second insulating film, and a Josephson junction formed by connecting the first superconductive thin film with the second superconductive thin film; wherein the magnetic shielding film is patterned to provide isolation for each of the plural superconductive circuits.
Furthermore, according to a packaging arrangement of the present invention, there is provided a superconductive circuit comprising: a superconductive circuit including; a substrate, a magnetic shielding film formed on the substrate, a first insulating film for electrical insulation on the magnetic shielding film, a first superconductive thin film formed on the first insulating film, a second insulating film formed on the first superconductive thin film, a second superconductive thin film formed on the second insulating film, and a Josephson junction formed by connecting the first superconductive thin film with the second superconductive thin film; and two superconductive thin films magnetically coupled with the superconductive circuit.
Moreover, according to another aspect of the present invention, there is provided a superconductive circuit comprising: a plurality of superconductive circuits each including; a substrate, a magnetic shielding film formed on the substrate, a first insulating film for electrical insulation on the magnetic shielding film, a first superconductive thin film formed on the first insulating film, a second insulating film formed on the first superconductive thin film, a second superconductive thin film formed on the second insulating film, and a Josephson junction formed by connecting the first superconductive thin film with the second superconductive thin film; and two superconductive thin films magnetically coupled with the plural superconductive circuits respectively; wherein the magnetic shielding film is patterned to provide isolation for each of the plural superconductive circuits.
Still further, according to a low-noise circuit system scheme of the present invention, there is provided a superconductive circuit device comprising: a first Josephson transmission line including; a first Josephson junction pair made of two Josephson junctions, first and second inductive elements connected to the first Josephson junction pair in a loop configuration to form a first superconductive loop, a second Josephson junction pair made of two Josephson junctions connected in a loop configuration for sharing the second inductive element to form a second superconductive loop, a third inductive element connected to the second Josephson junction pair in a loop configuration on the second superconductive loop, an inductive element having an input terminal for signal input and an output terminal for signal output, Josephson junctions respectively connected to the input and output terminals of the inductive element, and direct current sources respectively connected to the input and output terminals of the inductive element; a second Josephson transmission line which is identical in structure to the first Josephson transmission line; a fourth inductive element which is connected to an input terminal of the first Josephson transmission line and magnetically coupled with the first inductive element; a fifth inductive element which is connected to an input terminal of the second Josephson transmission line and magnetically coupled with the third inductive element; a first terminal resistor disposed at an output terminal of the first Josephson transmission line; and a second terminal resistor disposed at an output terminal of the second Josephson transmission line.
Still further, according to a scheme for attaining a high amplification factor on simultaneous signal input in the present invention, there is provided a superconductive circuit comprising: a first Josephson transmission line including; a first Josephson junction pair made of two Josephson junctions, first and second inductive elements connected to the first Josephson junction pair in a loop configuration to form a first superconductive loop, a second Josephson junction pair made of two Josephson junctions connected in a loop configuration for sharing the second inductive element to form a second superconductive loop, a third inductive element connected to the second Josephson junction pair in a loop configuration on the second superconductive loop, an inductive element having an input terminal for signal input and an output terminal for signal output, Josephson junctions respectively connected to the input and output terminals of the inductive element, and direct current sources respectively connected to the input and output terminals of the inductive element; a second Josephson transmission line which is identical in structure to the first Josephson transmission line; a fourth inductive element which is connected to an output terminal of the first Josephson transmission line and magnetically coupled with the first inductive element; a fifth inductive element which is connected to an output terminal of the second Josephson transmission line and magnetically coupled with the third inductive element; a sixth inductive element connected to an input terminal of the first Josephson transmission line; a seventh inductive element connected to an input terminal of the second Josephson transmission line; and a signal input terminal connected to a terminal of the sixth inductive element and to a terminal of the seventh inductive element.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.